1. Field of the Invention
The present invention relates to a new kind of pipe realized with conventional reinforced concrete having an evenly distributed steel wire reinforcement, as well as to a method for its manufacture on an industrial scale. Pipes realized with conventional reinforced concrete having an evenly distributed steel wire reinforcement (known as T.A.D. pipes) are articles of manufacture made with conventional reinforced concrete in which the reinforcement structure is extremely distributed, homogeneously arranged and constituted by steel wires having an extremely small diameter (0.5-1 millimeters). This structure allows the obtainment of an extremely high mechanical strength of the whole.
2. Description of the Prior Art
The manufacture and the characteristics of T.A.D. pipes are disclosed in British Pat. No. 1.584.844 in a detail manner.
In this prior patent, even if the technological problem of the manufacture of T.A.D. pipes has been resolved, there was still not resolved the problem of weaving bundles of wires from one to the other of the ends of the pipes in a continuous and automatic way, in order to realize rapidly and economically a longitudinal reinforcement of the pipes themselves.
As a matter of fact, the previous British patent describes clearly the difficulties connected with the set up of a longitudinal reinforcement, that was indeed expressely excluded, because even if the problem of the transversal strength or resistance to the burst of the pipe was resolved completely, the longitudinal strength or flexion strength of the pipe was relied upon bundles of wires spirally wound in an inclined manner with respect to the axis of the pipe, thus developing a reinforcement action both longitudinal and transversal or circumferential. As a matter of fact, the inclination determines a component of the strength in a longitudinal sense and another component in a transversal sense, so that the inclined reinforcement may, theoretically perform the function of giving strength in both the directions.
With this the problem of the automatic (and consequently rapid and economical) laying down of the circumferential reinforcement had to be considered as resolved. However, subsequent tests have brought in evidence noteworthy technological inconveniences, and serious limitations in the characteristics of the pipes. In particular the bundles of inclined circumferential wires are wound by means of a carriage that travels with a continuous to and fro movement, along the axis of the mandrel. The ratio between the travelling speed of the carriage and the rotational speed of the mandrel determines the inclination of the spiral formed by the bundle of wires. Clearly, at the end of each travel, i.e. in proximity of the ends of the mandrel, the carriage must gradually slow down to a stop, to stay still for a short time interval sufficient so that the mandrel makes at least half a turn, so that the wires are anchored in the concrete already projected on the mandrel itself, and then to start with a gradual acceleration the travel in the opposite sense. It derives therefrom that, in proximity of the ends of the pipes, the inclination of the spiral goes to zero and consequently the longitudinal component of the reinforcement vanishes. On the other hand the transversal reinforcement becomes excessive and needlessly encumbrant, up to the point that the compacting of the concrete is made difficult, and in that area it will be of low quality. Indeed, several experimental pipes manufactured in this way according to the British patent previously mentioned, gave origin to annular cracking near the ends, for the combined effect of the lack of longitudinal reinforcement and the scarce strength of the concrete.
It has been tried to obviate to this inconvenience with the introduction of supplementary reinforcement, constituted by ribbons of metal network, wound on the ends of the pipes between the several layers of bundles of wires. This solution was however unsatisfactory because (apart from the further worsening of the characteristics of the concrete, made still more serious by the stiffness of the networks, that with their settling determine the creation of voids in the concrete material) it has reintroduced in the manufacture cycle a manual operation, to be performed several times during the manufacture, with the mandrel at a stop, so that the principle that had brought to the selection of a inclined spiral reinforcement choice is no more valid. The inclination of the reinforcement, in practice, cannot go beyond a certain angular value: this because to an increase of the inclination there corresponds an increase of the length of the mandrel occupied by the bundle of wires for each revolution of the mandrel itself, and consequently also an increase of the number of wires to be wound, and an increase of the width of the band of concrete to be projected immediately over the wires. These two parameters (number of wires and width of the band of concrete that it is possible to project in a uniform thickness and extremely thin) determine the maximum inclination of the spiral reinforcement that, in practice results of about 20.degree. with respect to the directrix of the pipe.
Once that the maximum inclination has been fixed also the maximum contribution that the inclined spiral reinforcement may provide to the longitudinal strenght is fixed, strenght that in other terms becomes a fixed percentage of the transversal strenght.
It results therefrom that pipes to be utilized under low pressure, and consequently having a reduced transversal reinforcement, result poorly reinforced also in a longitudinal sense, and consequently they must be realized in short sections for limiting the flexure effects. On the other hand when elements having a greater lenght are to be produced, necessarily a reinforcement must be adopted that, in a transversal sense, is adequate for high pressures, with the consequence that pipes destined to lower pressures show a needless high strength.
These technological constraints will be better understood from the following practical example.
Let us consider a pipe having a diameter of 600 millimeters, and having a wall thickness of 50 millimeters. For producing it in the commercially accepted lenght of 5 meters, it is necessary to adopt a longitudinal reinforcement (distributed on the whole annulus of the transversal section of the pipe) of at least 6 square centimeters of iron. This reinforcement, owing to the 20.degree. inclination of the spiral, will correspond to a lenght of 74 centimeters of longitudinal section, so that for every linear meter there will be about 22.5 square centimeters of reinforcement, that correspond to a iron percentage of about 4.5% in volume with respect to the wall section. The composite material will have a strenght of about 150 kilograms/square centimeters and the pipe will fail at about 25 atmospheres. It is not possible to manufacture pipes having a diameter of 600 millimeters, and a lenght of 5 meters for lower pressures.
On the other hand if a pipe has to be manufactured having a diameter of 600 millimeters and a wall thickness of 50 millimeters, that fails under about 10 atmospheres, a strength of the composite material will have to be guaranteed of 60 kilograms/square centimeters and consequently an iron percentage of 1.5%, i.e. a reinforcement of 7.5 square centimeter for linear meter. The annulus resulting from the transversal section of the pipe will be traversed by only 2 square centimeters of iron useful to the longitudinal strength, and it will not be possible to manufacture the tube with a lenght greater than 3 meters so that it bears the flexure effects.
It is clear that (apart the technological difficulties and the economical burden connected with the production of pipes having different lengths for different pressures), the market would not accept neither pipes of different length nor pipes of the same length, but oversized in respect of the strength to internal pressure and consequently having a non competitive cost. It is clear therefore that the solution of the inclined spiral reinforcement from a practical standpoint is not prone to be transferred from the theoretical realm to the practical industrial one.
In Italian Pat. No. 545746 dating back to 1955, wherein a first attempt for producing a conventional reinforced concrete pipe on a rotating mandrel is disclosed, a transversal reinforcement was adopted constituted of steel rods that had to be placed by hand on the pipe. The reinforcement constituted of large diameter rods as disclosed in this patent, does not constitute in any way a distributed reinforcement in the sense meant in the patent (published Italian application No. 52836 A/76), because the strength provided by the rods results concentrated in discrete points in the wall of the pipe. Consequently such a solution, that corresponds to the teaching of the conventional art, is not applicable for a pipe having a distributed reinforcement in the sense meant by the present invention.